The use of organotitanium compounds as crosslinking catalysts in cathodically depositable electrodeposition coating materials has been recommended for many years. For example, DE-A 27 52 198 describes organic titanium compounds which are in the form of chelates and which, in addition, can be reacted at least in part with cationic coating binders. The products obtained are not suited to practical application. Thus, simple organotitanium compounds such as tetraalkyl orthotitanates can only be dispersed in an aqueous medium with great difficulties, using wetting agents, and are not stable to hydrolysis. Water-soluble chelate compounds of low molecular weight, in the course of the conventional methods of ultrafiltration, are uncontrollably removed from the deposition coating material. Moreover, the obtained products of reaction of such organotitanium compounds with cationic coating binders are similarly not stable to hydrolysis.
Proposals for overcoming the above described difficulties are set forth in AT Patent 392 647 U.S. Pat. No. 4,973,613 corresponding to and in AT Patent 390 451 corresponding to U.S. Pat. No. 4,973,613, which relate to titanium chelate compounds of .beta.-hydroxyalkylamine compounds, preferably having a relatively high molecular mass. The ability to use these products for combinations with cationic coating binders, however, are severely limited, since they lead to the development of a pronounced structural viscosity.
U.S. Pat. No. 5,132,341 corresponding to AT Patent 393 510 corresponding to U.S. Pat. No. 5,132,341, describes titanium-containing epoxy resin-amine adducts as pigment paste resins for cathodically depositable coating binders. However, "aging," for example in the case of too little of the coating material being used in the deposition tank, results in hydrolysis products which cause a settling of the pigments, surface defects in the deposited films, and a reduced reactivity of the catalyst component restricting their use.
The titanium phenolates obtained in accordance with AT Patent 390 621 corresponding to U.S. Pat. No. 4,942,196 are more stable to hydrolysis. However, these products cause severe discoloration of the baked films, as a result of which it is impossible to use the products in many applications, for example in light-colored primers.
Moreover, all of the known organotitanium compounds require, for their processing, considerable quantities of organic auxiliary solvents, which cause environmental problems and therefore their use is not favored.
AT Patent 396 373 describes a process for the preparation of organotitanium compounds which are stable to hydrolysis, which contain small proportions of organic auxiliary solvents and are based on alkylation products of phenols, modified with glycidyl compounds. However, such products also have a negative effect on the stability on storage of pigment pastes. The viscosity behavior of these pigment pastes is anomalous, i.e., at relatively high solids content their viscosity rises rapidly (thickening of the pigment pastes) while at a lower solids content the pigments tend increasingly to settle.
It has now been found that the demand for pigment pastes which are free of heavy metals, stable on storage and ready for use in the so-called two-component process can be met if the cross-linking catalysts used in cathodically depositable electrodeposition coating materials are organotitanium compounds which are obtained by a two-stage transesterification of tetraalkyl orthotitanates using specially selected hydroxyl compounds.